Also see Fuel Mileage
After filling the gas tank, the fuel guage may only read 3/4 full. Whilst not operationally normal, this is pretty common and can be fixed.
The fuel level sensor can be removed while the tank is in the car. Remove the center console to gain access to the sensor.
Once removed, inspect the sensor for overall integrity. The float drives a linear potentiometer which signals the analog fuel gauge. The part marked "fuel level sensor" in the above picture is a switch that activates the "empty" light. Corrosion can lead to failure of one or both of these components of the sensor. You can access the potentiometer by removing its metal cover.
Always be safe when working with exposed gasoline. Never use a torch, lighter, soldering iron, or smoke cigarettes while working on the fuel system. Make sure to reinstall the sensor and gasket whenever you leave the car, whether you are done working on it or not.